Combustors for burning a volatile fuel with air are used e.g. in heat engines in which the exhaust gases should contain very small amounts of unburnt hydrocarbons of CO and NO.sub.x. Simultaneously it is desired to obtain a high degree of efficiency i.e. the heat losses to the stack should be small.
Hitherto the air for the combustion of the fuel has been supplied as a rather cool, non-preheated flow of primary air and a flow of preheated secondary air, the preheating being obtained by exchanging heat with the exhaust gases prior to their passing to the stack.
The use of non-preheated primary air involves a heat loss and the amount of primary air should therefore be kept low relative to the amount of secondary air used. However, such proportion between the primary and secondary air will cause a longer flame and a longer time for the combustion to be completed. In order to obtain low emissions of NO.sub.x the combustion should be cooled down as soon as possible by exchanging heat with the medium to be heated, which in case of a heat engine is the working medium of said engine.